As reactive acrylic adhesives, anaerobic adhesives, second generation acrylic adhesives (SGA) and thermosetting adhesives have recently been known.
An anaerobic adhesive will be cured when the adhesive composition is pressed between the materials to be bonded, so that air is shut off. Accordingly, a portion of the adhesive squeezed out when pressed and being in contact with air, will not be cured. Such an adhesive is widely used since it is of one-pack type and thus excellent in the operation efficiency.
A second generation acrylic adhesive is of a two-pack type, but it is not required to accurately measure the two liquids, and it can be cured in from a few minutes to a few tens minutes at room temperature by very rough measurements, followed by mixing, or by mere contact of the two liquids. Thus, it has excellent operation efficiency. Besides, it has high peel strength and impact strength, and curing of a squeezed out portion is also excellent. Therefore, this adhesive is widely employed.
A thermosetting adhesive comprises an acrylate monomer and/or a methacrylate monomer, and an organic peroxide, as the main components, and is a one-pack type adhesive which will be cured by thermal decomposition of a peroxide.
However, a product having a steel sheet or a galvanized steel sheet bonded thereto is frequently subjected to baking finish to obtain a final product, and when conventional acrylic adhesives are subjected to such a high temperature as baking finish (e.g. at a level of from 160.degree. to 180.degree. C.), the bonded portion is likely to deteriorate, whereby the bond strength tends to substantially decrease.
Various proposals have been made for preventing heat deterioration of acrylic adhesives. For example, Japanese Unexamined Patent Publications No. 173174/1983 and No. 174476/1983 propose to use highly heat resistant polysiloxane or ethylene-acrylate rubber as an elastomer component to prevent the heat deterioration. On the other hand, Japanese Unexamined Patent Publication No. 129372/1987 proposes to incorporate an epoxy acrylate to an acrylate component and/or a methacrylate component to prevent the heat deterioration. Further, Japanese Unexamined Patent Publication No. 147477/1983 discloses that the heat deterioration can be prevented by using a butadiene elastomer and a phosphorus-containing compound in combination in the absence of a free organic acid.
However, the above methods are all directed to preventing the heat deterioration of the adhesive resins themselves, and there has been no instance in which prevention of heat deterioration is attempted in connection with the materials to be bonded. In many cases, a steel sheet or a galvanized steel sheet has chemical conversion treatment applied to improve corrosion resistance as a prime coat for baking finish. In such chemical conversion treatment, a metal to be treated is chemically reacted with a corrosive solution (a chemical conversion bath) prepared under a specific condition, whereby a strongly adherent insoluble corrosion product will be formed on the metal surface, which serves as a corrosion resistant layer, as a prime coat for painting or as a lubricating coat for plastic processing. A phosphate treatment, a chromate treatment or an oxalate treatment may be mentioned as such a surface conversion treatment.
When a metal having such treatment applied is used as a material to be bonded, it has been impossible by conventional techniques to prevent the deterioration of the bond strength due to heat deterioration of the adhesive.
In a case of a urethane type coating material or adhesive, it sometimes happens that free isocyanate groups react with moisture to form carbon dioxide gas, and the gas bubbles cause a deterioration of the surface condition or a deterioration of the bond strength. To prevent such gas bubbles, it has been proposed to incorporate a certain specific zeolite (Japanese Examined Patent Publication No. 37331/1973). However, in an acrylic adhesive as in the present invention, free isocyanate groups as present in a urethane-type adhesive do not exist, and no gas bubbles will form. Accordingly, it has not been known to incorporate zeolite to an acrylic adhesive.
It is an object of the present invention to solve the above-mentioned problems.